F. Liu

Bio: F. Liu is an academic researcher from University of Brasília. The author has contributed to research in topics: Borophene. The author has an hindex of 1, co-authored 1 publications receiving 2 citations.

Robust Topological Nodal-Line Semimetals from Periodic Vacancies in Two-Dimensional Materials.

Abstract: A nodal-line semimetal (NLSM) is suppressed in the presence of spin-orbit coupling unless it is protected by a nonsymmorphic symmetry. We show that two-dimensional (2D) materials can realize robust NLSMs when vacancies are introduced on the lattice. As a case study we investigate borophene, a boron honeycomb-like sheet. While the Dirac cones of pristine borophene are shown to be gapped out by spin-orbit coupling and by magnetic exchange, robust nodal lines (NLs) emerge in the spectrum when selected atoms are removed. We propose an effective 2D model and a symmetry analysis to demonstrate that these NLs are topological and protected by a nonsymmorphic glide plane. Our findings offer a paradigm shift to the design of NLSMs: instead of searching for nonsymmorphic materials, robust NLSMs may be realized simply by removing atoms from ordinary symmorphic crystals.

Vacancy-engineered flat-band superconductivity in holey graphene

Abstract: A bipartite lattice with chiral symmetry is known to host zero energy flat bands if the numbers of the two sublattices are different. We demonstrate that this mechanism of producing flat bands can be realized on graphene by introducing periodic vacancies. Using first-principle calculations, we elaborate that even though the pristine graphene does not exactly preserve chiral symmetry, this mechanism applied to holey graphene still produces single or multiple bands as narrow as ~0.5eV near the Fermi surface throughout the entire Brillouin zone. Moreover, this mechanism can combine with vacancy-engineered nonsymmorphic symmetry to produce band structures with coexisting flat bands and nodal lines. A weak coupling mean-field treatment suggests the stabilization of superconductivity by these vacancy-engineered narrow bands. In addition, superconductivity occurs predominantly on the majority sublattices, with an amplitude that increases with the number of narrow bands.

Symmetry-enforced nodal lines in the band structures of vacancy-engineered graphene

Abstract: We elaborate that single-layer graphene with periodic vacancies can have a band structure containing nodal lines or nodal loops, opening the possibility of graphene-based electronic or spintronic devices with novel functionalities. The principle is that by removing carbon atoms such that the lattice becomes nonsymmorphic, every two sublattices in the unit cell will map to each other under glide plane operation. This mapping yields degenerate eigenvalues for the glide plane operation, which guarantees that the energy bands must stick together pairwise at a boundary of the Brillouin zone. Moving away from the Brillouin zone boundary causes the symmetry-enforced nodal lines to split, resulting in accidental nodal lines caused by the crossings of the split bands. Moreover, the density of states at the Fermi level may be dramatically enhanced if the nodal lines crosses the Fermi level. The nodal lines occur a variety of vacancy configurations even in the presence of Rashba spin-orbit coupling. Finally, our theory also explains the nodal loops surrounding the entire Brillouin zone of a chevron-type nanoporous graphene fabricated in a recent experiment.

Vacancy-engineered nodal-line semimetals

Abstract: Abstract Symmetry-enforced nodal-line semimetals are immune to perturbations that preserve the underlying symmetries. This intrinsic robustness enables investigations of fundamental phenomena and applications utilizing diverse materials design techniques. The drawback of symmetry-enforced nodal-line semimetals is that the crossings of energy bands are constrained to symmetry-invariant momenta in the Brillouin zone. On the other end are accidental nodal-line semimetals whose band crossings, not being enforced by symmetry, are easily destroyed by perturbations. Some accidental nodal-line semimetals have, however, the advantage that their band crossings can occur in generic locations in the Brillouin zone, and thus can be repositioned to tailor material properties. We show that lattice engineering with periodic distributions of vacancies yields a hybrid type of nodal-line semimetals which possess symmetry-enforced nodal lines and accidental nodal lines, with the latter endowed with an enhanced robustness to perturbations. Both types of nodal lines are explained by a symmetry analysis of an effective model which captures the relevant characteristics of the proposed materials, and are verified by first-principles calculations of vacancy-engineered borophene polymorphs. Our findings offer an alternative path to relying on complicated compounds to design robust nodal-line semimetals; one can instead remove atoms from a common monoatomic material.

Translational dependence of the geometry of metallic mono- and bilayers optimized at semi-ionic supports. The cases of Pd at γ-Al2O3(110), monoclinic ZrO2(001), and rutile TiO2(001).

Abstract: A simplified computational scheme for “deposition” of a metallic (Pd) monolayer on semi-ionic supports is considered. An arbitrary selection of the initial position of the deposited layer relative to the surface of the support cannot provide the best solution for an optimal atom–atom binding. The energy and/or geometry changes were checked relative to a slide of the mono- and bilayers in parallel to the support layer and re-optimization. Two types of possible consequences are shown at the slide length of 1–3 A for γ-Al2O3(110), monoclinic ZrO2(001) (or m-ZrO2), and rutile TiO2(001) (or TiO2 below). First consequence option: either the Pd monolayer completely collapses after the shift/optimization, or it yields a new geometry (at γ-Al2O3(110) and m-ZrO2) at different steps. Second consequence option: the shift leads to a loss of the Pd stabilization while the ordered monolayer's geometry is retained (at TiO2 and m-ZrO2). The extent of such destabilization varies depending on the strength of the interaction with the support. It is thus recommended to verify Pd stabilization energy by sliding/optimizing the monolayer along one or two monolayer directions in order to avoid its energy underestimation. The activity of such monolayers for modelling catalysis is demonstrated using H2O dissociation in the Pd monolayer/rutile system.